Recent studies of prostate cancer suggest an enhanced risk associated with prior vasectomy. The incidence of prostate cancer in middle-aged men in western Washington has increased by about 60% over the past decade, and vasectomy is a widely used method of contraception in this geographic area. If findings from recent studies are confirmed, this could represent a major public health problem. The primary aims of the proposed study are to determine whether or not vasectomy is related to an increased risk of prostate cancer, and to assess whether there is evidence of a possible latent effect of vasectomy. To study the relationship between vasectomy and prostate cancer, we propose a population-based case-control study of 700 cases and 700 controls. Eligible cases will be men 40-64 years of age who are diagnosed with incident prostate cancer between January 1, 1993 and December 31, 1996 and who are residents of King county in northwestern Washington State. Cases will be ascertained through the Seattle-Puget Sound SEER Cancer Registry. Controls will be identified through random digit dialing, and will be frequency matched to cases on age (same 5-year group) and county of residence. Study subjects will be interviewed in person. Information will be obtained on history of vasectomy, reproductive and sexual history, medical history, demographic variables, and health behaviors such as smoking and the prevalence of use of screening procedures for the early detection of prostate cancer. Data analysis will include descriptive, stratified, and multivariate analyses. The relative risk of prostate cancer in relation to vasectomy will be estimated, after accounting for the potential confounding effects of other exposure variables.